Since a lithium ion secondary battery has a high energy density, the battery is used in compact devices for consumers such as mobile phones and personal computers, and recently, its application in not only these compact devices but also in automobiles has been accelerated.
A lithium ion secondary battery generally has a positive electrode and a negative electrode, and additionally it is conventional to place a separator for the purpose of electrical insulation between these electrodes. As a separator for a lithium ion secondary battery, for example, a microporous sheet made of a polyolefin-based resin is used.
The separator made of the microporous sheet plays a role in maintaining safety of a lithium ion secondary battery, when a short circuit takes place inside the battery, by closing a pore of the separator due to a shutdown function of the separator so as to prevent a lithium ion movement at the short circuit part and accordingly eliminating a battery function of the short circuit part. However, when a battery temperature is, for example, over 150° C. due to momentarily generated heat, the separator may drastically contract and the short circuit part between a positive electrode and a negative electrode may expand. In this case, a battery temperature may reach an abnormally overheated state of several hundred degrees centigrade or higher, and therefore, there is a problem in safety.
Thus, as a means to solve the aforementioned problem, a technique is suggested in Patent Literature 1, where an inorganic oxide porous membrane which contains an inorganic oxide filler having insulation performance is formed on a surface of a positive electrode, a negative electrode or a separator constituting a lithium ion secondary battery.
In addition, Patent Literature 2 discloses a lithium ion secondary battery which has a porous membrane having a thickness of from 0.5 to 20 μm obtained by using highly heat resistant α-alumina particles having an average particle diameter of from 0.1 to 5 μm as an inorganic oxide filler for use in such an inorganic oxide porous membrane.